Have We Been Wrong About Protein?

The world of sports nutrition is constantly advancing, yet there are some practices that have stood the test of time (and research). Within the endurance field, one such example is the comparative lack of emphasis on protein consumption compared to power and resistance based sports, where it is broadly acknowledged for its role in muscle hypertrophy and strength development. However, new research has brought into question the previous substrate recommendations for endurance athletes with regards to recovery, training adaptation and performance. Carbs have long since been hailed King or Queen, but is protein about to make a move for the crown?

What is the role of protein in exercise metabolism?

Dietary protein provides a source of amino acids, essential for the repair and remodelling of skeletal muscle. In response to exercise, both during and after, muscle protein synthesis and breakdown occurs. The myofibrillar proteins responsible for contractile force generation are remodeled (increasing strength and overall mass), and mitochondrial proteins, responsible for producing the energy to power these muscles, are also synthesised. Whilst an endurance athlete may not be wanting to increase their muscle mass, the strength and energy production elements are clearly relevant to performance.

In order to ensure that muscle protein synthesis and remodeling occurs, instead of breakdown, essential amino acids obtained from dietary protein must be in plentiful supply. This is particularly important to note for endurance athletes. Studies have repeatedly shown that around 6% of total energy cost of exercise is used in amino acid breakdown, which is further elevated during endurance exercise, particularly of branched chain amino acids (BCAAs) crucial for muscle synthesis. During endurance exercise, BCAAs can be broken down as a direct fuel source or transported to the liver to produce glucose as an indirect fuel source.

What endurance athletes need – requirements vs recommendations.

This new study makes a point of highlighting that there are discrepancies between protein requirements and protein recommendations within current sports nutrition discourse.

Protein requirements are ‘the minimum daily protein intake necessary to satisfy the metabolic demands of the body which includes the maintenance of body composition’.

Protein recommendations are ‘protein strategies to optimise performance in athletes by facilitating training adaptation and/or accelerating recovery’.

In a 2017 comprehensive study of endurance athletes across varying disciplines, from rowing to ice skating and ultra-endurance pursuits, recorded an average habitual protein intake of 1.5g/kg of body mass per day. This is relatively high  compared to the general recommendation of 0.8g/kg, but like all elements of nutrition, the needs of an individual are highly personalised, none more so than individual athletes. This general recommendation was calculated using nitrogen balance methodology, where participants consume diets with varying levels of protein (where almost all nitrogen in the body is found). The amount of nitrogen consumed through the individual diets is calculated and compared to each participant’s nitrogen excretion. Where the nitrogen ingestion and excretion was equal, this was considered to be the required protein needs for an individual. This method has been largely criticised for underestimating true protein requirements of all individuals, not simply athletes.

Instead, this new study takes a more contemporary approach using an indicator amino acid oxidation (IAAO) method which specifies how much of a certain amino acid (such as lysine or leucine) an individual needs in their diet. Amino acids are the building blocks of protein, and you may be familiar with the concept of a ‘complete protein’ – animal based sources of protein such as meat and dairy are complete proteins, however many plant based sources of protein, such as beans and legumes, are considered incomplete proteins as they do not contain the full spectrum of amino acids required by the body. This is how the IAAO method works, when the body does not have a sufficient supply of a key amino acid, it is unable to fully utilise the others,  instead they get broken down and released as waste.

This technique relies upon stable isotope tracing technology; participants are given a diet with very low levels of the key amino acid being tested, as well as an amino acid marked with an isotope to act as the ‘indicator’. As just explained, due to the lack of the specific amino acid being tested, the indicator amino acid cannot be used by the body to build protein and so gets oxidised. The carbon dioxide that is consequently released is then measured through breath and blood samples. The level of key amino acid in the diet is gradually increased until there is a plateau or cessation in the oxidation of the indicator amino acid. This is known as the ‘breakpoint’ and shows the minimum requirement of the key tested amino acid required within the diet to allow protein production.

The IAAO technique is behind the research conducted at the University of Toronto with the most notable results in relation to protein intake for endurance athletes. Across studies, endurance exercise markedly increases the protein requirement on both training and recovery days, above the average 1.5g/kg per day previously reported by endurance athletes. Endurance-trained men completed a 20km treadmill run, and the mean protein requirements of that training day was calculated to be 1.83g/kg per day of protein – this is over twice the RDA for the general population! This supports the notion that protein requirements increase in response to endurance exercise, likely to replenish the BCAAs lost and provide sufficient amino acids for muscle protein repair and remodelling.

Whilst we don’t advocate a low carbohydrate diet, an interesting outcome from UoT’s research was mean protein requirement increasing further, to 1.95g/kg, when endurance-trained men performed under conditions of low CHO availability. Further proving the fact that amino acid oxidation increases when in a glycogen-depleted state, and muscle protein synthesis cannot occur during periods of low energy availability. Lesson being; eat your carbs, or your body will eat your muscles. What’s more, when measurements were taken on a passive recovery day, 24 hours after intense endurance exercise, the mean protein requirements rose to 2g/kg, reinforcing the aforementioned needs for repair, replenishment and remodelling.

But what about female athletes?

Unfortunately, like much of sports science and medicine, research performed on and for women is woefully limited. Yet from the research out there, the results are even more significant than for their male counterparts. A study of female endurance athletes in team sports had their mean protein requirement calculated at 1.71g/kg.  When the same trial was performed on male team sport endurance athletes, the estimated requirement was only 1.40g/kg per day. Worth nothing was that all female participants were in the luteal phase of their menstrual cycle during the trial, when protein needs are understood to be highest. This is due to higher levels of oestrogen decreasing amino acid oxidation, and increasing the utilisation of body fat as an energy source. Compare this to the follicular phase, where oestrogen levels are lower, it is supposed that amino acid oxidation decreases, and so do protein requirements in accordance. A menstrual cycle periodisation approach to protein requirements has not been substantiated in research, but goes to show the potential of women that stands to be unlocked when studies focus on the female athlete.

So exactly how much protein do I need and when?

Looking first at intra-endurance exercise nutrition, there has been no meaningful evidence to support that co-ingesting protein with your carbohydrate fuelling source yields any additional performance benefit. So for as long as you’re consuming the optimum quantity of CHO for your body and intensity of activity, you won’t be missing out on any marginal gains.

In the recovery window immediately following exercise, studies have shown the positive impact of consuming 20g protein within the first 30-60 minutes, particularly on myofibril muscle protein synthesis, and consuming 45g protein to result in elevation of mitochondrial muscle protein synthesis. Further studies have gone on to show that the type of complete protein ingested makes little difference, but all have an equal anabolic effect.

When considering athletes with limited recovery time, be that from a multi-day event to cycling the Grand Tours, protein can be a valuable assistant to carbohydrates when it comes to optimising the process. Carbohydrates are the body’s primary fuel source, and we are generally only able to store 400-600g of glycogen in the body at any one time, which is depleted during endurance exercise. Normally, replenishing muscle and liver glycogen stores is achievable within 24 hours. But there isn’t that time between one stage of a race finishing, and the next one starting, so targeted nutritional intervention is key. Carbohydrates will always be the primary substrate for glycogen store replenishment, but their co-ingestion with protein and/or free amino acids can increase glycogen resynthesis rates during this period. This is notably the case when athletes struggle to have a high/optimum CHO intake following physical exertion.

In a similar vein, strategic protein nutrition has a practical application when it comes to improved tolerance to periods of intensified training. This has been repeatedly shown in several studies with trained cyclists, where during 4-7 day blocks of intensified training coupled with an increase in protein intake to 3g/kg of body weight demonstrated a better maintenance of endurance performance. Not only this, but there were also ‘favourable changes’ to creatine kinase levels (a marker of muscle damage), psychological symptoms of stress and immune status.

Carbs are still wearing the crown, but protein is in the picture.

There is no denying that protein’s previously underrated status in the endurance field has done neither it, nor athletes any favours. The array of studies cited within this piece highlight that having a less attentive approach to the macronutrient is a thing of the past, and a conscious consumption stands to have as many benefits to the performance and recovery of endurance athletes as it has long since been known to for their resistance trained counterparts. However, we should not be surprised that carbohydrates remain the most important substrate with regards to promotion of muscle glycogen restoration and enhanced overall performance.